After a term has ended your Canvas course will automatically be concluded. As a result of this you will not be able to add people to the Canvas course after the quarter has ended.

To change this, go into your course Settings

Under Course Details, click on the end date calendar and select some date in the future.

With the end date changed, click on the Users can only participate in the course between these dates box to override the term dates

Once you have changed the settings, click Update Course Details to save the changes

You will now be able to add people to your course again

Note: Once you have added the people to your course you should uncheck the Users can only participate in the course between these dates setting so that the course doesn't continue to show up as an active course.